Coral that is good for beginners: zoanthids, mushrooms (avoid jawbreakers and Bounces), favia, star polyps (caution needs to be taken as these can be invasive), nepthia trees, montipora, trumpet coral, chalice coral (must be less that $60 for about a 1 inch frag)
Moderate level corals: expensive chalice, frogspawn, hammer coral, galaxy coral, brain corals, plate corals, bubble coral, blastos (only solid red, or red and green) acans, cynarina (green only) torch coral (green) Duncan coral
Hard Level corals: scolymia, acanthrophyllia (meat coral), cynarina (reds, yellows, pinks), red gonoporia (red flowerpot), gold torch coral, jawbreaker mushrooms, blastos (the ones with yellow are the hardest), high end zoas (stratospheres and speckled kraks are some of the hardest),
Expert Level corals: gorgonians, all non-photosynthetic sponges, sun coral, green flowerpot, carnation trees, elegance coral,
I did not list many sps corals (except monti) but price typically helps dictate some of the difficulty of these. most of the expensive ones are slow growers and can be lost easy